* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

.container {

    width: 300px;
    height: 170px;
    position: relative;
    margin: 250px auto;
    overflow: hidden;
}

.rainbow {
    width: 200px;
    height: 200px;
    position: absolute;
    top: 70px;
    left: 50px;
    border-radius: 50%;
    box-shadow:
        violet 0px 0px 0px 5px,
        indigo 0px 0px 0px 10px,
        blue 0px 0px 0px 15px,
        green 0px 0px 0px 20px,
        yellow 0px 0px 0px 25px,
        orange 0px 0px 0px 30px,
        red 0px 0px 0px 35px
}